For the purpose of recovering information contained in a signal transmitted from one station to another, it is necessary at the receiving station to tune or lock the receiver to the frequency of the transmitted signal so that the demodulation process will accurately track the signal of interest. Where the information to be recovered is contained in a narrow bandwidth signal, the center frequency of which is unknown, the ability of the receiver to recover the transmitted signal will depend upon its initial task of locating the signal and then locking onto the previously unknown center frequency. While bandpass filtering schemes are sometimes useful in this respect, they are not always reliable, particularly where the frequency uncertainty region is so wide as to effectively reduce the signal-to-noise ratio of the output of the filter to an unacceptable level.